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Cutting capacity is usually described in terms of clean cut and maximum cut thickness, and the two are worth distinguishing. Clean cut is the thickness a machine handles with a good edge finish and reasonable speed, while maximum cut is the thickest material the machine will get through at all, usually slower and with a rougher edge. Buying with your typical material thickness in mind, rather than the thickest job you might occasionally face, generally gives a better day-to-day result.
Duty cycle is one of the most misunderstood figures on a welder's spec sheet, yet it tells you more about real-world usability than the headline amperage does. It's expressed as a percentage over a ten-minute period at a given output, so a machine rated at 30% duty cycle at its maximum amperage can run for three minutes out of every ten at that setting before it needs to rest and cool.
Portability and power supply matter as much as the process itself. A stick welder will run from a generator or a domestic supply in places a gas bottle can't easily follow, while MIG and TIG set-ups need a gas cylinder and, for anything beyond light-gauge work, a heavier electrical supply. Workshop layout, the materials you weld most often, and how frequently the machine needs to travel are all worth weighing up before settling on one process.
Welding produces fume made up of fine particulates and gases, and the composition varies depending on the process, the filler material and any coatings on the base metal. Fume rises from the arc and, without adequate control, can build up in the breathing zone of anyone working nearby, which is why extraction is treated as a core part of workshop set-up rather than an optional extra.
Ambient temperature and airflow around the machine also affect real-world performance. A welder working in a hot, poorly ventilated space, or one that's been boxed in against a wall with no clearance for its cooling fan, will hit thermal cut-out sooner than the same machine used with proper clearance in a cooler environment. Keeping vents clear and giving the unit room to breathe protects both the duty cycle you paid for and the components inside.
Switching speed, the time the filter takes to darken once it detects an arc, is worth checking against how you actually work rather than assuming faster is always better for every budget. For http://kuhpedia.de/index.php?title=User:MauricioMorrisse most general fabrication and repair work, a mid-range auto-darkening helmet with a sensible shade range covers the vast majority of jobs comfortably.